Problem: William is 4 times as old as Emily and is also 21 years older than Emily. How old is William?
Answer: We can use the given information to write down two equations that describe the ages of William and Emily. Let William's current age be $w$ and Emily's current age be $e$ $w = 4e$ $w = e + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $w$ is to solve the second equation for $e$ and substitute that value into the first equation. Solving our second equation for $e$ , we get: $e = w - 21$ . Substituting this into our first equation, we get the equation: $w = 4$ $(w - 21)$ which combines the information about $w$ from both of our original equations. Simplifying the right side of this equation, we get: $w = 4w - 84$ Solving for $w$ , we get: $3 w = 84$ $w = 28$.